1. Technical Field
The present invention relates to a horizontal electric field type liquid crystal display device using an fringe field switching (FFS) or the like mode whereby pixel electrodes and a common electrode are disposed opposing each other on a planarization layer with an insulator interposed and a method for manufacturing such device.
2. Related Art
Many of the liquid crystal display devices in use are of the vertical electric field type which has a pair of transparent substrates with electrodes and so on formed on their surfaces and a liquid crystal layer that is sandwiched between the pair of substrates, and displays various kinds of information by rearranging the liquid crystals via application of voltage to the electrodes on the two substrates. Such vertical electric field type liquid crystal display devices are generally twisted nematic (TN) mode, but since this has the problem of a narrow viewing angle, various improved vertical electric field type liquid crystal display devices with vertical alignment (VA) mode, multidomain vertical alignment (MVA) mode or the like have been developed.
On the other hand, liquid crystal display devices that, unlike the vertical electric field type liquid crystal display devices described above, are in-plane switching (IPS) mode or FFS mode, whereby electrode pairs constituted of a pixel electrode and a common electrode are provided on one of the substrates only, are also known.
Of these, the IPS mode liquid crystal display device deploys the electrode pairs on the same layer, and makes the direction of the field that is applied to the liquid crystal almost parallel with the substrates, so as to rearrange the liquid crystal molecules in a direction parallel with the substrates. Therefore, the IPS mode liquid crystal display device is also called a horizontal electric field type liquid crystal display device, and has the advantage of providing a viewing angle that is extremely broad in comparison to the vertical electric field type liquid crystal display devices described above. With the IPS mode liquid crystal display device however, because the electrode pairs are provided on the same layer in order to apply an electric field to the liquid crystal, there is the problem that the liquid crystal molecules, which are positioned above the pixel electrode, are inadequately driven, and this results in lowering of the transmittivity and other aspects.
What might be termed diagonal electric field type FFS mode liquid crystal display devices have been developed (see JP-A-2001-235763 and JP-A-2002-182230) in order to resolve the problems of the IPS mode liquid crystal display device. In such FFS mode liquid crystal display devices, the pixel electrodes and common electrodes for applying the electric field to the liquid crystal layer are disposed in differing layers, each with an insulator interposed.
Such FFS mode liquid crystal display device has a broader viewing angle and higher contrast than the IPS mode liquid crystal display device, and furthermore can be driven with low voltage, as well as having higher transmittivity. Thus it has the feature of enabling bright displays. In addition, with the FFS mode liquid crystal display device there is also the advantage that the area of overlap, viewed from above, between the pixel electrodes and common electrodes, is larger than in the IPS mode liquid crystal display device, which has the side-effect that a larger auxiliary capacitance is generated and there is no need to specially provide an auxiliary capacitance wire.
However, with the FFS mode liquid crystal display devices disclosed in JP-A-2001-235763 and JP-A-2002-182230, there is the problem that, because the portions of the pixel electrodes that overlie the TFT elements and common electrode line have an irregular form, in other words a form with multilevels, misalignment of the liquid crystal molecules occurs at such multilevel portions, so that such multilevel portions constitute regions that essentially do not contribute to the display, and therefore the aperture ratio falls. An approach that is practiced to address such problem is to employ a planarization layer such as is used in the VA type or MVA type liquid crystal display devices mentioned earlier, and to dispose the pixel electrodes, common electrodes and so forth on such planarization layer, in order to eliminate the formation of multilevels in the surfaces of the pixel electrodes that overlie the switching elements, common wire and so forth (see JP-A-2001-283540 and JP-A-2007-226175).
Nevertheless, with the FFS mode liquid crystal display devices disclosed in JP-A-2001-283540 and JP-A-2007-226175, after formation of the regular TFT elements, common electrode line and so forth, a planarization layer constituted of photosensitive material, for the purpose of planarizing, is formed and the first electrodes, inter-electrode insulator and second electrodes are formed on the surface of such planarization layer. Therefore the problem arises that, compared with earlier FFS mode liquid crystal display devices that do not use a planarization layer, the number of photolithographic, layer-forming and dry etching process increases, so that more processes are needed for manufacturing, with the result that productivity falls. In addition, because photolithographic and etching methods are employed to form contact holes in the passivation layer via the contact holes formed in the planarization layer, the width of the contact hole mouths is large, so that the opening of the liquid crystal display device obtained is low.